The present invention relates to a method for manufacturing a flexible tubular underwater pipe for transporting fluids of hydrocarbon or other type, intended for use in particular at great depths. It relates also to a pipe obtained according to the method that is the subject of the invention.
It relates more particularly to the pipes of the unbonded type described notably in the following normative documents published by the API (American Petroleum Institute):                API 17J Specification for Unbonded Flexible Pipe.        API RP 17B Recommended practice for Flexible Pipe.        
Also, in order to manufacture these flexible tubular pipes, a leakproof tubular structure is provided, consisting essentially, from inside to outside, of a leakproof sheath or pressure sheath, a pressure vault made of a profile wire wound with short pitch around the sheath, and at least one layer of armor wires wound in a helix with long pitch around said pressure vault. In the present application, the term “with short pitch” corresponds to a helix angle of between 70° and 90°, whereas the term “with long pitch” corresponds to a helix angle less than 60°. The hydrocarbon is intended to flow inside the leakproof sheath. The pressure vault takes up the radial forces which are exerted on the pipe, whereas the layer of armor wires takes up the axial pulling forces.
Furthermore, the layer of armor wires is usually covered with an outer sealing sheath, so as to protect the abovementioned various underlying layers from the surrounding water.
Some flexible pipes do not include a pressure vault, but have, in this case, crossed reinforcements wound with a helix angle close to 55°, this particular angle enabling the reinforcements to take up both radial and axial forces.
These pipes, when they are installed in an underwater environment, are subjected to external pressures which can be higher than the internal pressure that prevails inside. Also, an axial compression may occur, which is known to those skilled in the art as the reverse end cap effect. This effect tends to shorten the pipe. These axial compression forces can reach a very high level. For example, a pipe transporting gas with an outer diameter of 300 mm installed at a depth of 2000 m can be subjected to an axial compression force of the order of 140 tonnes when it is depressurized. The pipe is then subjected to an external pressure of approximately 200 bar, whereas the internal pressure is around 1 bar.
The reverse end cap effect thus has the tendency to axially compress the flexible pipe, and thereby, shorten its length, which tends to increase its diameter. This phenomenon also has the effect of causing an inflation of the pulling armor plies, that is to say a radial excursion of the armor wires. In the case where the outer sheath of the pipe is leakproof, the hydrostatic pressure prevailing outside the pipe effectively opposes the inflation of the pulling armor layers. By contrast, if the outer sheath is no longer leakproof, for example following an accidental tear, the water invades the interior of the outer sheath and the latter is no longer pressed by force at the hydrostatic pressure against the layers of armor wires and therefore no longer opposes the inflation. Consequently, in the absence of an additional means designed to limit this inflation, the wires that make up the pulling armor plies may buckle according to a radial mode, which may cause an irreversible local deformation of said layers of armor wires, which take on a “bird cage” shape. The flexible pipe is then denatured locally.
One known solution that makes it possible to reduce this risk of radial buckling in “bird cage” form consists in winding in a helix with short pitch, around the layers of pulling armor wires, aramid fiber-reinforced tapes, said tapes exhibiting a high mechanical tensile strength on their longitudinal axis. This anti-inflation layer specifically makes it possible to limit the inflation of the layers of pulling armor wires. These tapes also exhibit a great deflection flexibility, which simplifies the operations of handling and of winding around the armor plies. Finally, given equal mechanical characteristics, they are much lighter than metal tapes, which makes it possible to reduce the weight of the flexible pipe.
Reference can notably be made to documents WO03/083343, FR2926347 and WO2008/135663, which describe such types of pipes.
However, this solution has a number of drawbacks. First of all, these aramid fiber-reinforced tapes are very costly. Furthermore, this solution does not totally solve the problem of lateral buckling of the armor wires, notably when the pipe is simultaneously subjected to variations of curvature, notably in proximity to the sea bed. In this area, the pipe can be stressed simultaneously by axial compression, by the reverse end cap effect, and deflection, by the movements of the floating support to which the pipe is connected. In these conditions, given that the armors no longer have the possibility of inflating freely according to a radial mode, it is still possible for them to buckle according to a lateral or circumferential mode.
Also, one problem that arises and that the present invention aims to resolve, is how to provide a flexible pipe which not only can be manufactured at an advantageous cost, but also which better withstands the reverse end cap effect.